1. Field of the Invention
The present invention relates generally to a field effect transistor and a high-frequency power amplifier having the same.
2. Description of the Related Art
Metal Semiconductor Field Effect Transistors which will be hereinafter referred to MESFETs, have been used for power amplifiers for portable telephones. The MESFETs of this type generally have a recess structure. However, in recent years, MESFETs having a self alignment structure capable of being operated at a low voltage in lower costs have been used. FIG. 9 shows a conventional MESFET structure having a self alignment structure having a short gate length.
As shown in FIG. 9, a p-type impurity layer 82 for suppressing a short channel effect is formed on a semi-insulating GaAs substrate (also hereinafter referred to as a "substrate") 81. In the surface region of the p-type impurity layer 82, an n-type active layer 83 is formed. On the n-type active layer 83, a Schottky gate electrode 84 is formed. In addition, n-type impurity layers (also hereinafter referred to as "intermediate layers") 86a, 86b are formed in the impurity layer 82 so as to sandwich the gate electrode 84 therebetween. Outside the intermediate layers 86a and 86b in the impurity layer 82, an n-type source region 88a and an n-type drain region 88b are formed.
In the MESFET of this structure, each of the source-side and drain-side intermediate layers 86a, 86b generally has a depth of about half of those of the source region 88a and the drain region 88b and a density of impurity substantially equal to those of the source regions 88a and the drain regions 88b to prevent the potential below the active layer from decreasing to suppress the short channel effect and to decrease the parasitic resistance to achieve a high transconductance and a low source-drain resistance. However, the drain breakdown voltage of the MESFET of this structure is about 6 V, so that it is not sufficient to use it for a power amplifier having a source voltage of about 3 V. Therefore, as shown in FIG. 10, there have been used structures wherein no drain-side intermediate layer 86b is formed and an active layer 83 is elongated to be connected to a drain region 88b to enhance the breakdown voltage (M. Nagaoka, et al., SSDM93, PD-2-1) and wherein a drain-side intermediate layer 86b is elongated (S. Murai, et al., IEEE GaAs IC Symp. Dig., pp139-142, 1992).
However, since these structures are asymmetric structures, there is a problem in that it is required to accurately carry out the mask alignment in the manufacturing process, so that the yield is considerably decreased and the manufacturing costs are increased.